A semiconductor Integrated Circuit (IC) device receives and transmits an electrical signal through its input/output port from/to an external device. The input/output port is connected to an input/output driver in a semiconductor device, wherein the driver is constructed such that a PMOS transistor and an NMOS transistor are connected in series between a positive power supply and a ground, and the gates of the PMOS transistor and the NMOS transistor is controlled by respective enable signals. For example, in order to output “1” to the input/output port, an enable signal of “0” is inputted into the gates of the PMOS and NMOS transistors. An electric current is delivered to the input/output port from a positive power supply connected to a source of the PMOS transistor. In order to output “0” to the input/output port, an enable signal of “1” is inputted into the gates of the NMOS and PMOS transistors. An electric current of the output port is discharged through a ground terminal connected to the source of the PMOS transistor.
As seen from the above, a drive capacity of the input/output driver of the semiconductor IC device is determined according to physical sizes of the PMOS transistor and the NMOS transistor (i.e., widths and lengths of the transistors), which constitute the driver. In comparison to currents required for internal operations of a semiconductor IC device, the input/output port, which has to output an electrical signal to the external devices, needs a greater current, and thus sizes of transistors used in the driver are significantly larger than those of transistors for internal circuits of the semiconductor IC device.
However, even if the driver is optimally designed, the current driving capacity may be insufficient or otherwise may be higher than needed, because there may be widely differing external devices connected to the semiconductor IC devices. In the case where the current drive capacity is insufficient, the speed of the device may suffer, and, in extreme cases, the external device cannot be driven. On the other hand, in cases where the current drive capacity is too high, the external device is provided with unnecessary electrical noise.